The invention relates to a method of monitoring the welding operation in a stud welding process and a device for carrying out the method.
When carrying out a stud welding process, in order to obtain the best possible welding quality, it is necessary to observe the set values of relevant welding parameters with corresponding accuracy. In the stroke ignition method and in similar methods which the present invention relates to, the parameters welding current, welding time, length of projection, stroke or lifting-off distance when drawing the arc, and dampening during immersion in particular are decisive for the quality of the welding connection between the stud and the workpiece.
Measuring and monitoring the electric parameters, namely, the welding current and the welding voltage, has been known and commonly applied for quite a long time, but particularly in case of the stroke ignition method, observing the mechanical parameters such as the length of projection, the lifting-off distance and immersion dampening, has only been made possible by pre-adjusting these parameters in the stud welding device or the corresponding welding head.
What is disadvantageous here, however, is that the movement of the stud of known stud welding devices inserted into the welding head is not monitored and thus deviations of the movements from predetermined set values are not detected. This means that a negative influence on the welding quality resulting from the fact that a predetermined length of projection or lifting-off distance was not observed, or deviations from the set value of dampening during immersion are not detected.
In order to solve this problem in the tip ignition welding process, the German patent DE 41 24 511 C1 proposes that the actual value of the immersing speed of the welding stud into the welding pool that is created at the point of welding of the workpiece be measured and compared with a set value for the speed obtained empirically. Deviations of the actual value from the set value which are outside a predetermined range of tolerances then show the insufficient quality of the welding connection that has been completed. As the immersing speed is the decisive parameter for the welding quality in the tip ignition welding process, the document DE 41 24 511 C1 proposes that this value be determined by measuring the time which the welding stud requires in order to cover a short distance Δs immediately before it is immersed into the melted mass of the workpiece.
In the stroke ignition welding process, however, the quality of a welding connection does not only depend on the immersing speed, but also on the dampening during immersion, which means that what is decisive is the time gradient of the movement of the stud or the time gradient of the stud speed from the point of time when it is immersed into the melt. Furthermore, it is desirable to monitor the observance of the predetermined parameters “length of projection” and “lifting-off distance”, because they have an influence on both the stud speed during the phase when it is immersed into the melt and on the behaviour of the electric arc.
From the documents DE 39 29 669 A1 and DE 43 07 325 A1, stud welding devices are known, respectively, whose aim it is to effect a movement of the moving part assembly (which comprises all moving parts that cooperate to perform the actual welding movement such as the stud holding device, the charging head, the feed cylinder, the feed piston, etc.) or of the stud holding device as exactly as possible and in line with a predetermined course. For this purpose, in the device according to DE 39 29 669 A1, a control magnet is connected with a path measuring device and an electronic control in order to thereby determine the actual value of the regulating distance of the armature of the control magnet, to compare it with a set value and to effect control of the movement of the armature with which the moving part assembly is connected. In the device according to the document DE 43 07 325 A1, the moving part assembly or the stud holding device of the welding gun is moved with the aid of a lifting magnet which acts as a controlled brake so that no additional damping units or devices are required. What is disadvantageous in these devices, however, is that insufficient welding quality of completed welds is not recognizable if—for whatever reasons—the course of welding deviates from an ideal or predetermined course.
For this reason, in the document DE 43 14 528 C2, it is proposed to continuously measure the time gradient of the movement of the stud towards the workpiece, at least during the entire phase of movement when the stud is immersed into the melt of the workpiece. Hereby, deviations of the measured course of movement or of the time gradient of the speed of movement determined therefrom from a predetermined set value for the course or the gradient can be detected.
In spite of this well-tried method, in practice, there was an increased demand for more exact and more detailed information about the quality of a weld, particularly in case of aluminium welding.